The invention concerns optical detector arrays, also known as focal plane arrays (FPAs).
The conversion of optical signals into electrical signals forms the basis for a wide range of useful functions. A focal plane array (FPA) is a semiconductor device that converts light impingent upon it into electrical charge. The optically generated electrical charge is collected at localized areas or pixels of the image focal plane. The pixels are arranged in N rows and M columns to form an N by M array. A CID (charge injected device) is one type of FPA used to convert optical signals to electrical signals. Another type of FPA device is a charge-coupled device (CCD), which is the most common type of an FPA. CCD""s were commercially brought to use in television cameras. Their use has since grown significantly. Exemplary applications of CCDs include security systems, scanners, cameras, fax machines, telescopes, machine vision systems and a wide range of imaging systems. Other FPA devices have a similar wide-ranging applicability.
CCD (and other FPA) devices may be silicon devices, while the modern trend is toward MOS (metal oxide semiconductor devices). CID devices inject a charge into an underlying semiconductor and that charge may be individually addressed and read out. A CCD device is another embodiment of an FPA in which the electronic charge collected in each pixel coupled to the adjacent pixel charge. The charge in each pixel is transferred to the edge of the array by shifting the charge along a row of column of the array where it is converted to a voltage. The individual pixel charge is coupled or moved to the edge of the device using voltages on a number of conductors that span the FPA in a manner similar to shifting data in a shift array. The array of pixels, i.e., individual picture elements, is typically formed from MOS (metal oxide semiconductor) capacitors. A typical pixel is in the range of 10 to 20 xcexcm square. As the underlying semiconductor devices become smaller, similar reductions in the pixel size of CCDs and other FPAs is possible. Arrays of the MOS capacitors are created on a substrate to form a typical CCD device. When a photon of sufficient energy strikes the depletion region of a MOS capacitor, it is absorbed and causes an accumulation of charge carriers, typically electrons, at the oxide-semiconductor interface. Extracting this result permits the determination of the quantity and intensity of light striking an element in the array.
With the use of optical filters and particular arrangements of FPAs, the FPAs can also be used to determine the wavelength of impingent optical energy, allowing, for example, the detection of color in addition to the intensity and location of light striking a FPA array. Electronic techniques are used to extract data from FPAs. The speed of extraction and the memory requirements for extraction comprise two major limitations to the size and general applicability of FPA arrays.
The basic electronic method for reading data from a CCD array involves a sequential application of voltage to gates of the MOS capacitors to transfer electrons from gate to gate in shift register fashion. A typical configuration is called a three-phase CCD
In a three-phase CCD, voltages on consecutive gates are controlled by three out-of-phase clock signals. A three stage process shifts the charge collected by one gate. The charge in each pixel is converted to a voltage as the charge reaches the edge of the array and the voltage is used as a measure of the light that was incident on an individual pixel. Repetition of this process many times completes transfer of one column of charge. For M columns, M repetitions are required. The overall speed of data retrieval therefore depends on the number of columns and the speed of the repetition that is required to shift the data to edge of the array. For example, retrieving data from common CCDs requires several milliseconds.
Other FPAs may be read out directly, i.e., they are charge collection devices instead of charge transfer devices. In such FPAs (including CIDs) individual pixels are accessed to determine their charge. For large FPAs or for applications requiring high speed data retrieval, the typical method for avoiding the step-wise column by column transfer of a CCD is to include a memory buffer arranged similarly to the FPA pixel array. The memory buffer accepts full frame input. Memory sizes can be quite large, however, adding expense and complexity. In addition, electronic access to large arrays of pixel data can also be slow compared to the potential for the rapid change in optical energy impingent upon the FPA.
According to an aspect of the invention, the charge of pixels in a FPA array is sampled by propagating an optical wave along a waveguide positioned to enable the evanescent electric field of the optical wave to interact with the pixel charges. An optical detector receives an optical wave altered by the respective pixel charges after the wave has traversed the array along a column or row.
In another aspect of the invention, an electric field is used to hold and release pixel charges. A direct optical memory is thereby realized. Pixel charge is held while an electric field maintains the separation of electron-hole pairs resultant from impingent optical energy upon pixels while under influence of the applied electric field. By maintaining the electric field, the pixel charges in an array may be kept and sampled numerous times. The memory is cleared when the applied electric field is reduced or removed.